A simple kinetic approach to derive the ecological dose value, ED50, for the assessment of Cr(VI) toxicity to soil biological properties

Three New Zealand soils of contrasting texture, organic matter content and cation exchange capacity (CEC) were amended with K2Cr2O7 solutions, spanning two concentration ranges, 0–5 μmol Cr(VI) g−1 soil and 0–50 μmol Cr(VI) g−1 soil. Samples were assayed for phosphatase, sulphatase and urease enzyme activities, and for basal respiration, microbial biomass C, dimethyl sulphoxide(DMSO)-reducing activity and denitrification, 3 and 60 d after amendment. Extractability of Cr(VI) from similarly amended samples was measured from 0 to 100 d. Cr(VI) proved to be strongly inhibiting of most of the biological properties and in most instances inhibition was explained by one or both of two simple Michaelis-Menten kinetic models. The first of these (Model 1) simulated fully competitive kinetics and the second (Model 2) simulated partially competitive kinetics. A single inhibition constant, similar to ED50 as conceptualized in previous studies, could usually be calculated for each property in each soil. The properties could be ranked in the following order of decreasing sensitivity to Cr(VI): denitrification > DMSO-reduction > sulphatase activity ≈ biomass C > phosphatase activity > urease activity > respiration. For the most sensitive property, denitrification, ED50 values range from 63 to 730 nmol Cr g−1 soil. Soil mineral surface area, organic matter content and CEC influenced the sensitivity of properties between soils. Although the extent of inhibition often diminished with time, the differences were generally much smaller than the observed decline in extractability of Cr(VI), indicating that a persistent, long-term inhibition, outlasting the Cr(VI) itself, had occurred.